1. Field of the Invention
This invention relates to a urea-formaldehyde condensation product and to a method for manufacturing the product. More particularly, this invention relates to a urea-formaldehyde condensation product suitable for use as a substitute for titanium dioxide as an opacifier in the manufacture of cellulosic products, and a method of manufacturing this product.
2. Description of Related Art
Urea-formaldehyde condensation products are well known and are put to a variety of uses. In particular, urea-formaldehyde pigments are useful as modifiers of cellulosic products such as paper. Large quantities of inorganic pigments such as talc, kaolin, calcium carbonate, zinc sulfide, clay, titanium dioxide, and the like are employed as fillers in the production of paper products. Ordinarily, such inorganic pigments are effective in the preparation of paper products having suitable properties of brightness, opacity, basis weight, softness, smoothness, finish, and ink absorption.
An additive for cellulosic products such as paper must perform the functions of an inorganic filler for which it is substituted if it is to gain wide commercial acceptance. Further, to be acceptable to the paper industry, a chemical additive for paper must be one which does not migrate from the base sheet to adjacently held absorbent material. Also, the additive must not cause "blocking", i.e., sticking together of adjacent paper layers when the paper is rolled or stacked sheet upon sheet.
Urea-formaldehyde condensation products are suitable as synthetic additives because they provide the above-described desired characteristics to paper and cellulosic products. Condensate products, also called condensation products, result from the reaction of, in this instance, urea and formaldehyde to form polymeric products. Combination of these monomer molecules with the elimination of water is known as condensation.
Suitable condensation products typically contain urea and formaldehyde in a range of relative proportions and may be prepared in accordance with many known methods. Condensation products may be useful in more than one technology. For example, see U.S. Pat. No. 3,981,845, wherein a urea-formaldehyde condensation product is described as suitable both as an additive for paper and a reinforcing filler for rubber.
Typical methods of making urea-formaldehyde condensate products are disclosed in U.S. Pat. Nos. 3,981,845, 3,909,348, and 4,367,171. Other methods of making urea-formaldehyde resins are known. For example, in a preparation method disclosed in U.S. Pat. No. 4,410,685, urea and formaldehyde are first reacted in a very acidic solution (i.e., pH less than about 2.5), then the reaction is completed under basic conditions. Such product typically is utilized as an adhesive or binder for wood-containing constructions.
Typically however, known preparation methods have drawbacks that make them less than satisfactory in commercial applications. In many known methods for preparing additives for paper, such as the method described in U.S. Pat. No. 3,981,845, a mixture of urea and formaldehyde or a urea-formaldehyde precondensate is polymerized. Precondensates typically are prepared in neutral or alkaline solutions. However, such solutions are not stable for extended periods, and must therefore be used promptly. Thus, known methods using such precondensates are unsatisfactory because failure to use precondensate essentially immediately results in a waste of solution and precludes preparation of precondensate days in advance of its intended use or in a location other than the immediate vicinity of its intended use. Further, use of urea and formaldehyde separately require separate storage and handling facilities and sources of supply, and entails other inconvenient aspects.